This invention relates to the construction of artificial islands.
Artificial islands have various uses, for example in supporting drilling platforms in oil-exploration and extraction, as foundations for lighthouses, and as copperdams or breakwaters. However, the cost of constructing such islands by depositing naturally-occurring materials can be extremely high because of the massive quantities of material which are required. The underwater angle of repose of most natural materials is extremely low, and there is also the problem of the scouring action of water which may not only erode the island but also lead to silting in nearby areas. Thus, it can be unacceptably costly to construct artificial islands except in areas of shallow and still water and, of course, islands are usually required in other locations.
It has recently been proposed to construct an artificial island by containing a sandy or other permeable material in an envelope or membrane of robust plastics material, and applying suction to the interior of the membrane to extract water and to reduce pressure in the membrane. Thus, the hydrostatic pressure of the surrounding water acts to rigidify the contained sand from which water has been extracted. Preliminary tests on these so-called "sand-islands" have given promising results, but difficulties can arise during placing of the sand in the membrane which is initially filled with water, as the sand tends to spread excessively before the angle of repose can be increased by water extraction and pressure reduction. In order to overcome these difficulties, which increase with the diameter of the membrane, water is drained from the sand and extracted from the membrane during sand filling. The water may be extracted through a central pipe or wellscreen and, to promote drainage flow to the pipe, vertically spaced layers of relatively coarse sand are introduced to provide generally horizontal drainage layers. Needless to say, the provision of these drainage layers, which are difficult to position accurately, increases the complexity and expense involved in producing the island.
It is an object of the present invention to provide an improved method of constructing sand islands, in which the aforementioned difficulties are reduced.